A thin film forming method for performing vapor deposition in a vacuum chamber to form a multilayer film is well known in the art.
This thin film forming method forms a multilayer film by alternately and repeatedly performing a vapor deposition process using several materials. By performing vapor deposition alternately and repeatedly using a material having a higher refractive index and a material having a lower refractive index, for example, a multilayer mirror or multilayer bandpass filter can be formed.
In order to form a multilayer film having desired optical characteristics, the film must be deposited so that each layer has a desired optical thickness. Optical thickness in this description is defined as the product of the refractive index and the film thickness.
In the conventional thin film forming method, a substrate to undergo vapor deposition and a substrate for monitoring the process are disposed in a vacuum chamber, and vapor deposition is performed on both substrates simultaneously. During the film forming process, measuring light is irradiated on the monitor substrate to monitor the amount of reflected light. When the amount of reflected light reaches a prescribed value, the vapor deposition is halted.
Each material in this process has a prescribed refractive index inherent in the material. Hence, if thin films of the same material have the same optical thickness, it is likely that the thin films have the same refractive index. When attempting to form multiple layers of thin films having the same optical thickness and using the same material in the thin film forming method of the prior art, the vapor deposition process is continued for each layer until the amount of reflected light reaches the same target value.
Let us consider a case for forming a multilayer bandpass filter having the wavelength transmitting property shown in FIG. 1(a), wherein 23 layers are alternately formed of titanium oxide (TiO2) having a refractive index of about 2.25 and silicon oxide (SiO2) having a refractive index of about 1.45. As shown in FIG. 2, of all 23 film layers, only the seventh and nineteenth TiO2 thin films have an optical thickness of 300 nm, while the rest of thin films have an optical thickness of 150 nm.
When a TiO2 film having a refractive index of about 2.25 has an optical thickness of 300 nm, the reflectance of the thin film is determined by the refractive index. Therefore, in the process for forming the 7th and 19th layers of the TiO2 film, vapor deposition is continued until the amount of reflected light reaches a prescribed target value corresponding to the reflectance. Further, when the TiO2 layer has an optical thickness of 150 nm, the thin film has a different reflectance. Therefore, in the process for forming the TiO2 layers having an optical thickness of 150 nm, vapor deposition is continued until the amount of reflected light reaches a target value corresponding to the different reflectance. When the SiO2 layer with a refractive index of about 1.45 has an optical thickness of 150 nm, the thin film has a reflectance different from the others. Accordingly, in the process of forming each layer of SiO2 film, vapor deposition is continued until the amount of reflected light reaches a target value corresponding to the reflectance.